<!doctype html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0"
        name="viewport">
  <meta content="ie=edge" http-equiv="X-UA-Compatible">
  <title>Document</title>

  <script defer src="https://unpkg.com/alpinejs@3.x.x/dist/cdn.min.js"></script>
</head>
<body>
<div x-data="{ open: false }"
     x-init="$watch('open', value => console.log(value))">
  <button @click="open = ! open">Toggle Open</button>
</div>

<div x-data="{ foo: { bar: 'baz' }}"
     x-init="$watch('foo.bar', value => console.log(value))">
  <button @click="foo.bar = 'bob'">Toggle Open</button>
</div>

<!--get old value-->
<div x-data="{ open: false }"
     x-init="$watch('open', (value, oldValue) => console.log(`current: ${value}, old: ${oldValue}`))">
  <button @click="open = ! open">Toggle Open</button>
</div>

<!--deep-->
<div x-data="{ foo: { bar: 'baz' }}"
     x-init="$watch('foo', (value, oldValue) => console.log(value, oldValue))">
  <button @click="foo.bar = 'bob'">Toggle Open</button>
</div>

</body>
</html>